Tire condition monitoring systems are known. By helping vehicle operators maintain vehicle tires at a proper inflation pressure, tire condition monitoring systems enhance vehicle safety, help reduce tire wear, and help reduce CO2 emissions as vehicles run more efficiently with properly inflated tires. Known tire condition monitoring systems may include tire-based sensors mounted on the inside of the tire (e.g., attached to the tire rim) for measuring tire conditions such as temperature and pressure. A tire-based transmitter operatively coupled to the tire-based sensor transmits a radio signal to a vehicle-based receiver unit. The transmitted signal may include both temperature and pressure information along with a unique sensor identification code that can serve as a tire identification code. The vehicle-based receiver unit monitors the tire condition signals, the received tire identification code, and controls a display device within the vehicle cabin to warn the vehicle operator when the sensed tire condition(s) are not within predetermined limits. The vehicle-based receiver unit can further indicate which tire location has an out-of-limit tire condition (e.g., overpressure or under-pressure) by associating the tire location with the received tire condition signals using the unique tire identification code.
Certain regulations have been proposed for tire pressure monitoring systems (e.g., by the United Nations Economic Commission for Europe (“UNECE”) Transportation Division and, in particular, regulation EC R64.02) applicable to certain classes of vehicles. These regulations require that the tire pressure monitoring system must provide a warning to the vehicle operator upon a predetermined percentage pressure loss (e.g., 20% pressure loss) within any one of the vehicle's tires. Since tire pressure is dependent on temperature, detection of a percentage pressure loss can prove difficult. In certain geographic area of the world, it is possible to have a large percentage variation of tire pressure simply due to ambient temperature variations particularly when combined with tire temperature increases during use. The four major factors that influence tire pressure are tire temperature, ambient temperature, ambient atmospheric pressure, and vehicle load conditions. Tire temperature varies during use, ambient temperatures vary over time and location, ambient atmospheric pressure varies according to altitude and weather, and vehicle load varies under use conditions. It is difficult to establish a cold tire pressure value during normal vehicle operation particularly when a vehicle operator inflates a hot tire to a recommended pressure value.
Some proposed systems automatically initiate a tire fill assist operation in response to a change of tire pressure when the vehicle is stationary. A problem occurs when a pressure change (e.g., pressure increase), occurs due to heating of the tire as a result of, for example, a hot disc rotor or running vehicle engine. In such a case, a pressure measurement could be potentially misleading due to temperature fluctuations. Since such tire fill assist systems may use audible and/or visual feedback for the vehicle operator, and improper initiation of the tire assist system could cause operator confusion.
In response to this, some tire condition monitoring systems implement the ideal gas law to detect pressure increases or decreases by monitoring the number of moles or mass of the air in the tire. The ideal gas law is PV=nRT, where P is pressure, V is volume, n is the number of moles, R is the ideal gas constant for the gas in question, and T is temperature. Assuming the volume of the tire to be constant, V and R can be ignored as constants and the equation reduces to n=P/T. Thus, using this simplified equation, the number of moles of air (n) in a tire can be monitored as being functionally related to the measured pressure (P) and temperature (T) information received from the TCM sensor. Examples of such systems are disclosed in Juzswik U.S. Pat. No. 6,612,165 entitled TIRE PRESSURE MONITORING SYSTEM WITH PRESSURE GAUGE OPERATING MODE FOR INDICATING WHEN AIR PRESSURE WITHIN A TIRE IS WITHIN A PREDETERMINED PRESSURE RANGE, issued Sep. 2, 2003, and Juzswik U.S. Pat. No. 8,818,619 entitled METHOD AND APPARATUS FOR DETERMINING TIRE CONDITION USING IDEAL GAS LAW, issued Aug. 26, 2014. The disclosures of both of these Patents are hereby incorporated by reference in their entireties.